This invention relates to a felt which is used in a press section of a papermaking machine, and more particularly to a papermaking press felt (hereinafter called a xe2x80x9cpress feltxe2x80x9d) having an improved water squeezing capability.
As shown in FIG. 13, a conventional press apparatus, used to squeeze water from a wet paper web in a papermaking process, comprises a pair of press rolls P, and a pair of press felts 12, which pinch a wet paper web W. When the press felts 12 and the wet paper web W are compressed between the press rolls P, water is squeezed from the wet paper web W and absorbed by the press felts 12.
Each of the press felts 12 comprises a base body (not shown) for maintaining strength, and batt layers (not shown) which are provided on both sides of the base body. The base body is integrated with the batt layers by needle punching.
FIG. 14, which is a partial, enlarged view of the press nip in FIG. 13, illustrates the movement of water which is squeezed from a wet paper web W. It does not illustrate the detailed structure of the press felts 12, however.
When the press rolls P rotate in the direction of the arrows in FIG. 13, the press felts 12 and the wet paper web W, which are sandwiched between the press rolls P are propelled in through the press nip in the direction of the arrows in FIG. 13.
As mentioned above, when the press felts and the wet paper web W are compressed, water is squeezed from the wet paper web W and absorbed in the press felts 12. However, within a short distance from the nip center to the delivery side of the rolls, the pressure applied to the wet paper web W and press felts 12 is rapidly released, and the volumes of the press felts 12 and the wet paper web W expand rapidly. As the expansion occurs, a pressure is applied to the press felts 12. Moreover, since the wet paper web is made of thin fiber, a capillary phenomenon also occurs. As a result of the pressure applied to the felts and the capillary action in the wet paper web, water which was absorbed in the press felts 12 shifts again to the wet paper web W. The phenomenon just described is known as the rewetting phenomenon, and is a problem in a conventional press apparatus. U.S. Pat. No. 5,372,876 discloses a felt designed to prevent rewetting. As shown in FIG. 15, in the felt 11, which comprises a base body 31 and batt layers 21 on both sides of the base body, a hydrophobic film 41, which is made of spunbonded filaments, is provided on the base body 31. This hydrophobic film 41 divides the felt into a press roll side layer and a wet paper web side layer. Even if pressure applied to the felt 11 is released rapidly, it is difficult for water which is absorbed in the press roll side layer to move to the wet paper web side layer. Accordingly, this felt 11 is supposed to be capable of suppressing rewetting.
Unexamined Japanese Patent Publication No. 8888/1991 discloses a felt in which a barrier layer which comprises a porous film is provided so that water absorbed in the barrier layer is prevented from moving to the wet paper web side.
In addition, U.S. Pat. No. 4,830,905 discloses a press felt in which a foam layer having independent bubbles is provided to prevent rewetting by holding water in the bubbles.
However, with the hydrophobic film having many small holes, and the porous film, used in the felts disclosed in U.S. Pat. No. 5,372,876 and Unexamined Japanese Patent Publication No. 8888/1991, suppressing the movement of water is difficult in practice, and good rewetting suppression may not be achieved.
U.S. Pat. No. 4,830,905 does not disclose how the water held in the bubbles is discharged, and accordingly there is a doubt about the practical effect of the press felt described therein.
The invention solved the above-mentioned problem by providing a papermaking press felt which has a wet paper web contacting surface and a roll contacting surface, the felt comprising a base body, a batt layer, and a rewetting prevention layer, in which the rewetting prevention layer has three-dimensional passages, each said passage comprising an opening rim, a wet paper web side opening and a roll side opening, the wet paper web side opening being larger than the roll side opening.
According to the invention, a rewetting prevention layer having passages with a three-dimensional structure prevents rewetting effectively.